This is a very surprising development. I'll provide a bit of backstory, the Yasukui shrine in Japan is a memorial for all of those who died serving the Japanese empire. Imperial Japanese soldiers were interred regardless of their behavior in war. I think around 14 war criminals are represented by the shrine.
Chinese leaders have made this a huge deal politically, and all progress with Japanese relations has hinged on their leaders not visiting the shrine (not exaggerating).
We'll have to see what happens now, but there are sure to be protests in China